Heretofore, coal has been gasified with stream to make methane, or fuel for power plants, and, in general, the uses of coal gas have paralleled those of natural gas. However, all previous conventional methods have usually generated coal gas by contacting coal with steam or hydrogen at high temperatures and pressures in accordance with the following equations: EQU C(amorph.) + H.sub.2 O(g) CO + H.sub.2 + some CH.sub.4
thereafter, CO is reacted with H.sub.2 as follows: EQU CO + 3H.sub.2 CH.sub.4 + H.sub.2 O
it has not been possible heretofore to achieve satisfactory rates of reaction when reacting coal with steam at relatively low temperatures, such that coal gasification would be practical at such low temperatures. A significant advantage, however, afforded by the present invention resides in its ability to carry out this reaction at much lower temperatures than had been previously thought possible. Thus, in accordance with the present invention, sulfur dioxide, which is normally considered a pollutant, can be used to promote reaction of steam and coal at significantly lower temperatures.
In the past, gasification of coal has been carried out by the reaction of steam on incandescent coke or coal at temperatures around 1000.degree. C. and higher where the reaction rate and equilibrium have been favorable. By using a higher molar volume ratio of steam relative to coal of 2:1, it has been possible to conduct this reaction at temperatures several hundred degrees lower.
Since the reaction of coal with steam is an endothermic one, which tends to cool off the coal or coke fed into the gasifier, it has been known to use calcium carbonate as a source of CO.sub.2 in the gasification process since such use accomplishes the following results: (1) it restores heat to the gasifier in view of the fact that the reaction of coal with CO.sub.2 is exothermic; and (2) it generates CO.sub.2 which is able to react with the coal to provide carbon monoxide, itself an intermediate in the preparation of methane.
However, it has not been previously possible, under conditions that would justify commercial operation, to conduct coal gasification at temperatures as low as those just upwards of 1200.degree. F., preferably temperatures approximating 1400.degree. F., at atmospheric pressure. By means of the present invention, however, low temperature gasification of coal with steam is now made possible.